1. Field of the Invention
The present invention relates to a process which is capable of efficient production of purified heavy components from coal tars which are suitable for use as starting materials in the production of carbon products. More particularly, the present invention relates to a method of purifying the starting materials for use in the production of carbon products comprising the following steps: distilling or flashing coal tar to obtain a heavy component composed of fractions having boiling points higher than a predetermined temperature; mixing said heavy component with a monocyclic aromatic hydrocarbon solvent to form a solution; subjecting the resulting solution to filtration or centrifugation so as to separate and remove the insoluble component; and distilling off the solvent. The purified heavy component obtained by the method of the present invention is particularly suitable for use in the production of high-performance carbon fibers.
2. Prior Art
High-performance carbon fibers are lightweight and have high strength and elastic modulus properties. Because of these features, high-performance carbon fibers are gaining increasing attention as components of composite materials for use in air-craft, sporting goods, industrial robots, etc. and a rapid increase in the demand for such carbon fibers is expected to occur in the future.
Most of the high-performance carbon fibers available today are produced from polyacrylonitrile (PAN) which is spun into filaments, rendered infusible in an oxidizing atmosphere, and is subsequently carbonized or graphitized in an inert atmosphere. Recently, it has been found that high-performance carbon fibers having characteristics which are equal to or better than the PAN-based carbon fibers can also be produced from inexpensive pitches, and several processes have been proposed for producing pitch-based carbon fibers.
According to known methods for producing pitch-based carbon fibers, preliminarily hydrogenated pitches are heat-treated to become suitable for use in subsequent spinning (see, for example, Japanese Patent Laid-Open Publication No. 196292/1983) or pitches are rendered suitable for use in spinning by heating them for a prolonged period at a relatively low temperature (see, for example, Japanese Patent Laid-Open Publication No. 86717/1978). The spinning pitches which are suitable for use as the starting materials for producing high-performance carbon fibers must be "mesophase" pitches the primary component of which is the mesophase which exhibits an optical anisotropy when observed under a polarized light.
The mesophase is a kind of liquid crystal that forms when heavy oils or pitches are heated. The optical anisotropy of the mesophase is believed to result from the laminar structure of the planar aromatic molecules developed by thermal polymerization When mesophase pitches are subjected to melt spinning, the planar aromatic molecules are aligned parallel to the filament axis under the stress which is exerted during passage through a spinning nozzle hole. This oriented structure is stable and maintained through subsequent stages of fiber production (i.e., rendering the filaments infusible and carbonizing the infusible filaments) such as to provide high-performance carbon fibers having good orientation.
The mesophase pitches can be produced from coal tars, tars as by-products of thermal cracking of naphtha, tars as by-products of thermal cracking of gas oils, and decant oils, but coal tars are used most commonly because of such advantages as low aliphatic contents, high aromaticity and high pitch yield.
Coal tars which are the heavy oils obtained as by-products of the dry distillation of coal contain very fine (0.1-0.3 .mu.m) sooty substances which are commonly referred to as free carbons. Coal tars also contain components having very high molecular weights.
When coal tars are heat-treated to produce mesophases, the free carbons are deposited on the mesophases such as to upset the laminar structure of the planar aromatic molecules in the mesophases. It is therefore impossible to make mesophase pitches having good orientation from coal tars containing the free carbons. In addition, the free carbons are solid substances which will not melt at elevated temperatures and can cause filament breakage during spinning or may produce low-strength fibers. It is therefore essential that the free carbons are eliminated at a certain stage of the process of making spinning pitches. The free carbons are insoluble in quinoline and can be removed from coal tars or pitches by subjecting quinoline solutions thereof to filtration or centrifugation, and this is a practice commonly employed on a laboratory scale. However, as already mentioned, the free carbons are in the form of very fine particles and can be filtered out only at very slow rates or can be separated by centrifugation with very low efficiency. It is therefore almost impossible to accomplish complete removal of the free carbons by carrying out filtration or centrifugation on an industrial scale.
The components of very high molecular weights in the coal tars become much higher in molecular weight as a result of thermal polymerization that occurs in the initial stage of heat treatment because of the high rate of reaction involved in the formation of mesophases or in carbonization. Such components of extremely high molecular weights will not only impair the homogeneity of the spinning pitches but also increase their melting points. Mesophase pitches start to soften at relatively high temperatures (250.degree.-300.degree. C.), so they have to be spun at significantly high temperatures (.gtoreq.300.degree.-350.degree. C.). It is said that most organics start to decompose in this temperature range. Therefore, the first requirement that should be met in order to produce carbon fibers of good quality is to prepare homogeneous spinning pitches which have minimal contents of those components of very high molecular weights which increase the melting points of the pitches.
Several methods for removing the free carbons and components of very high molecular weights (the two will hereunder be collectively referred to as "unwanted component") from coal tars by filtration or centrifugation have been proposed: in one method, aromatic and aliphatic solvents are used at specific mixing proportions (as in Japanese Patent Laid-Open Publication No. 78201/1977); in another method, hydrocarbons having BMCI values within a certain range are used as solvents (as in Japanese Patent Laid-Open Publication No. 28501/1977); and in still another method, solvents having a characterization factor within a specific range are used (as in U.S. Pat. No. 4,292,170). The essence of these methods is the same in that the ability of the solvent to dissolve coal tars is adjusted to the proper range. Since the solvent employed in these methods is either a mixture of two or more solvents or a light oil which is a complex mixture, the mixing proportions of the solvent components or the solubilizing ability of the solvent must be closely controlled when the solvent is recovered or put to another use.
The unwanted components may be eliminated from pitches without using solvents; for example, the pitches are directly subjected to filtration under heating (as in Japanese Patent Laid-Open Publication No. 142820/1975); or the pitches are subjected to filtration under heating after they are heat-treated to form small amounts of mesophases (as in Japanese Patent Laid-Open Publication No. 136836/1983). These methods are effective for the purpose of eliminating the unwanted components to produce homogeneous spinning pitches. However, if one wants to separate the free carbons from the pitches by direct filtration, the rate of filtration is very slow and its efficiency is extremely low since the free carbons are in the form of very fine particles (0.1-0.3 .mu.m). Even if the pitches are subjected to filtration under heating after they are heat-treated to form small amounts of mesophases, the resulting mesophases also exist as small spheres with a diameter in the order of a few microns. In addition, the mesophases are composed of molecules which are similar to those making up the non-mesophased isotropic components, and the latter works as a swelling agent for the mesophases such that, under heating, the mesophases either dissolve or swell and are thus softened to such an extent that the efficiency of filtration is markedly reduced.
U.S. Pat. Nos. 4,578,177 and 4,575,412 disclose a process for removing unwanted components from pitches consisting a step of heating the pitches to 350.degree.-500.degree. C. to give 10-30% by weight of mesophases due to condensation polymerization of components having very high molecular weights, dissolving so heat-treated pitches in an aromatic hydrocarbon solvent, and then removing insoluble components from the resulting solution. In this process, unwanted components are removed by bonding free carbons to the surface of mesophase particles produced by condensation polymerization of components having very high molecular weights. However, this process leads to a reduced efficiency of filtration in terms of the removal of insoluble components by filtration from the solution in which heat-treated pitches are dissolved in an aromatic hydrocarbon solvent.
Under the circumstances described above, it is desired to develop a method which is capable of efficient removal of the unwanted components from coal tars on an industrial scale at a certain stage of the process for producing spinning pitches suitable for use as starting materials in the production of carbon fibers.